1. Field of the Invention
The present invention relates to an electrophotographic image formation apparats capable of forming high quality images stably for an extended period of time, using an electrophotographic photoconductor comprising a substrate, a photoconductive layer provided on the substrate, and a protective layer formed directly or via an intermediate layer on the photoconductive layer.
2. Discussion of Background
Conventionally varieties of electrophotographic image formation apparatus have been known. A representative example of such an electrophotographic image formation apparatus is illustrated in FIG. 10.
The operation of the electrophotographic image formation apparatus shown in FIG. 10 is as follows:
A drum-shaped photoconductor 1 is rotated in the direction of the arrow, so that the surface of the photoconductor 1 is uniformly charged by a charger 2. A light beam which has been modulated in accordance with image signals to be recorded is applied to the charged surface of the photoconductor 1 by an exposure unit 3, so that latent electrostatic images corresponding to the image signals are formed on the surface of the photoconductor 1. The thus formed latent electrostatic images are then developed to visible toner images by a development unit 4. The toner images are then transferred to a copy paper 9 by an image transfer charger 5. The toner-image-bearing copy paper 9 is separated from the photoconductor 1, and the toner images are fixed to the copy paper 9 by an image fixing unit 10, whereby a hard copy is obtained.
After the above image transfer, the toner remaining on the surface of the photoconductor 1 is removed by a cleaning unit 7, and the residual charges on the surface of the photoconductor 1 are quenched by a quenching lamp 8. Thus a series of copy making steps is completed.
Various types of photoconductors are in general use as the above-mentioned photoconductor, for instance, a photoconductor comprising an electroconductive substrate, and a photoconductive layer formed thereon, which comprises as the main component selenium or a selenium alloy, for example, Se--As, Se--Te, and Se--As--Te; a photoconductor comprising an inorganic photoconductive material such as zinc oxide or cadmium sulfide, and a binder agent in which the inorganic photoconductive material is dispersed; a photoconductor comprising an organic photoconductive material (OPC) such as poly-N-vinylcarbazole or azo pigments; and a photoconductor comprising an amorphous silicon.
Fundamental characteristics required for the photoconductor 1 are the following characteristics (A) to (D):
(A) chargeable in the dark to an appropriate potential; PA1 (B) minimum dissipation of charges in the dark; PA1 (C) quick dissipation of charges when exposed to light; and PA1 (D) having sufficient mechanical and chemical resistances for practical use in electrophotographic process.
In view of the above characteristics (A) to (D) required for the photoconductor 1, the above-mentioned amorphous-silicon based photoconductor has high surface strength, high mechanical durability (for instance, in the contact portions in the development unit, image transfer unit, and cleaning unit), and excellent photosensitivity. However, the amorphous-silicon based photoconductor has the shortcomings that the chargeability thereof is low, the production cost is high, the resistance to chemicals, in particular, to ozone (O.sub.3), NO.sub.x, and composite materials formed from ozone (O.sub.3), NO.sub.x, and other materials contained in the atmosphere, is weak, and the surface resistance of the photoconductor decreases by the moisture in the air, which causes the formation of abnormal images, called "image flow".
The above-mentioned Se-based photoconductor has excellent photosensitivity and chargeability and has been conventionally most widely used. However, it has the shortcomings that the surface of the photoconductor is easily scratched and tends to produces images with non-printed stripes or black-striped images.
Recently organic photoconductors (OPC) have been widely used because of the advantages over other photoconductors that the production cost is low, almost no pollution problems are caused by the production thereof, and there is more freedom in the designing thereof in comparison with other photoconductors. However, organic photoconductors have the shortcomings that the surface hardness thereof is low because they are made of organic materials, and therefore the surface thereof is easily abraded, so that the potential of the charged surface is decreased by such abrasion and images with non-printed stripes or black-striped images are formed.
The provision of a protective layer directly or via an intermediate layer on the photoconductive layer has been proposed to eliminate the shortcomings of the Se-based photoconductor and organic photoconductors with respect to the mechanical durability thereof, thereby improving the durability to the mechanical stress or load applied thereto in or outside a copying machine.
For instance, an organic film is provided on the surface of a photoconductor in Japanese Patent Publication 38-015466, an inorganic oxide is provided on the surface of a photoconductor in Japanese Patent Publication 43-014517, an adhesive layer is provided on the surface of a photoconductor, and an insulating layer is overlaid on the adhesive layer in Japanese Patent Publication 43-027591, an amorphous silicon layer (a-Si layer), an a-Si:N:H layer, or a-Si:O:H layer is overlaid on the surface of a photoconductor by the plasma CVD method, or by the light CVD method in Japanese Laid-Open Patent Applications 57-179859 and 59-058437.
Varieties of films with high hardness consisting of carbon or comprising carbon as the main component, which are referred to as a-C:H film, an amorphous carbon film or a diamond-like carbon film, have recently been used as protective layers for photoconductors.
For instance, a protective layer composed of amorphous carbon or hard carbon is provided on the surface of a photoconductive layer in Japanese Laid-Open Patent Application 60-249155; a protective layer composed of a diamond-like carbon is provided on the top surface of a photoconductor in Japanese Laid-Open Patent Application 61-255352; an insulating layer with high hardness comprising carbon as the main component is provided on a photoconductive layer in Japanese Laid-Open Patent Application 61-264355; a protective layer produced by glow discharge, which is composed of an amorphous hydrocarbon film and contains at least atoms such as a nitrogen atom, an oxygen atom, a halogen atom, and an alkali metal atom, is provided on an organic photoconductive layer in Japanese Laid-Open Patent Applications 63-220166, 63-220167, 63-220168, and 63-220169.
Se-based photoconductors and organic photoconductors, which are improved with respect to the surface hardness thereof and have excellent resistance to abrasion, have been available at a relatively low cost by the above-mentioned methods.
However, when these photoconductors are repeatedly used in the electrophotographic process, the residual potential of the photoconductors, which is the potential at a portion of the surface of the photoconductors which have been exposed to light after a first charging, tends to increase temporarily or with time over a long period of time, although the durability to the abrasion is in fact increased, so that abnormal images are produced. The result is that the overall durability of these photoconductors cannot be improved by any of the above-mentioned methods.